1. Field of the Invention
This invention relates to a process for preparing an aqueous dispersion of higher fatty acid zinc salt. More particularly, the invention relates to a wet direct precipitating process for preparing a low viscosity aqueous dispersion of higher fatty acid zinc salt having a small particle size in high yields.
2. Description of the Prior Art
An aqueous dispersion of a metallic soap has been heretofore produced by first preparing a metallic soap and then dispersing the metallic soap in water in the presence of a surfactant. For industrial use, the metallic soap is produced mainly by a wet double decomposition process. In this process, an aqueous solution of the appropriate metal salt is added to an aqueous solution of sodium salt of a fatty acid, thereby precipitating the water-insoluble metallic salt. The process by-produces a large amount of water-soluble inorganic sodium salt. Accordingly, it is necessary that, after the reaction, the resulting metallic soap is fully washed with a large quantity of water to remove the inorganic sodium salt from the metallic soap. Moreover, as mentioned hereinabove, an aqueous dispersion of a metallic soap is produced by first preparing a metallic soap in this manner and then dispersing the metallic soap in water in the presence of a surfactant by means of a high performance agitator or a homogenizer. Under these circumstances, the conventional process for the preparation of aqueous dispersion of a metallic soap involves many steps and the production cost is inevitably expensive. In addition, the resulting dispersion not only comprises a metallic soap which has a large particle size and poor dispersibility but also has a high viscosity.
Other processes for the preparation of metallic soaps such as a dry direct fusion process or a wet direct precipitating process are also known. In the dry direct fusion process, a higher fatty acid is directly reacted with a metal compound at a high temperature. The process has a drawback that the resulting metallic soap suffers from thermal deterioration. The wet direct precipitating process is a process in which a fatty acid is first dispersed in water, and then a metal oxide or hydroxide is added to the dispersion so that the fatty acid reacts directly with the metal oxide or hydroxide. The process has an advantage that no water-soluble salt is by-produced contrast to the wet double decomposition process. The wet direct precipitating process has a further advantage that high temperature reactions are not needed so that the resulting metallic soap does not suffer from thermal deterioration, contrast to the dry direct fusion process.
However, according to the known wet direct precipitating process, it is difficult to complete the reaction between a higher fatty acid and a metal oxide. Therefore, the known process employs the metal oxide in excess so that the unreacted free higher fatty acid does not remain in the reaction mixture after the reaction. In consequence, the resulting aqueous dispersion of metallic soap has a low purity of the metallic soap.